1. Field of the Invention
This invention relates to a particle accelerator. More particularly, this invention relates to a particle accelerator having an accelerator guide, a particle source for injecting particles into the accelerator guide, a source of high frequency energy, a coupling element for introducing high frequency energy from the source of high frequency into the accelerator guide for energy exchange with and acceleration of the particles, and a delivery end for delivering pulses of particles. Still more particularly, this invention relates to an automatic control system for driving a particle accelerator, preferably a linear accelerator.
A linear accelerator of the type here contemplated is adapted to receive electrons at relatively low velocity, to accelerate the received electrons to high energy levels, and either to deliver the accelerated electrons as high velocity electrons or to direct them onto a target to generate X-rays.
2. Description of the Prior Art
Particle accelerators such as linear accelerators are used today in a number of different applications such as radio therapy, radiography and sterilization. In most of these applications the particle accelerator is used to generate X-rays.
It is very important that a constant dose rate output from the accelerator be achieved over both a short period of time, such as during a specific therapy treatment, as well as a long period of time such as day to day during successive treatments. It is also very important that the accelerator deliver electrons at uniform energy and power levels at each selected setting of the controls, which are associated with the particle source and the source of high frequency energy.
One of the leading causes of possible variation in the output from an accelerator is the change in particle output amplitude resulting from a mismatch between the operating frequency of the accelerator and the frequency of the driving signal applied thereto. This mismatch can result from dimensional changes in the accelerator structure or differential expansion in different parts of the accelerator structure thereby resulting in a change in its operational frequency.
According to U.S. Pat. No. 3,965,434, the desired relationship between the driving frequency source and the accelerator can be maintained by an automatic frequency control which tunes the frequency of the accelerator driver source to the operational frequency of the accelerator. The particle output amplitude from the accelerator is measured by a signal derived from an ionization chamber wherein current pulses are sampled to obtain an output signal representative of modulated particle (X-ray) pulses. This signal is amplified and applied through the automatic frequency control aspect of the system to a stepping pulse generator wherein a signal is added to minimize power modulation effects of the accelerator RF source and synchronously demodulated to produce a frequency error voltage and stepping pulses having rates which are proportional to this error voltage. The stepping pulses are directed to the drive of a stepping motor to step the tuner of the driver source to the point where the particle output amplitude is maximized. The automatic frequency control does not compensate for energy deviations due to the shift of components.
In U.S. Pat. No. 2,887,580, there is disclosed another variable output control for a linear accelerator. In this control, a pulsed radio frequency power supply system is employed to introduce energy into a wave guide. The frequency is controlled by a frequency adjusting circuit, and the output energy of the wave guide is continuously measured or monitored. A mixing circuit operates to combine a control impulse corresponding with the frequency of the measured output energy level with an impulse corresponding with the frequency variation of the power supply. The resultant impulse is then applied to the frequency adjusting circuit to correspondingly correct the frequency of wave guide input energy, while the pulse rate is simultaneously adjusted in response to monitored changes in the wave guide output power in order to maintain output power at a desired level. Despite this output control, the form of the output pulses may change due to a drift of the components of the circuit.